French President Emmanuel Macron said he was not ruling out sending western ground troops to Ukraine to achieve Europe's goal of defeating Russia.
"We are convinced that the defeat of Russia is indispensible to security and stability in Europe," Mr Macron said after a meeting in Paris of two dozen European leaders to discuss Ukraine.
Russia, he said, was showing a "more aggressive attitude not just in Ukraine but in general".
While there was "no consensus" on sending western ground troops to Ukraine, "nothing should be excluded", Mr Macron said.
"We will do whatever it takes to ensure that Russia cannot win this war."
He said a new coalition would be set up to supply Ukraine with "missiles and bombs of medium and long range to carry out deep strikes".
There was a "broad consensus to do more and quicker", Mr Macron said.
"Nothing can be excluded to achieve our objective. Russia cannot win that war."
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German Chancellor Olaf Scholz and Polish President Andrzej Duda were among the leaders of about 25 European countries at the conference.
Other states sent ministers, with UK Foreign Secretary David Cameron present, along with representatives from the US and Canada.
Mr Macron said earlier as he opened the conference at the Elysee Palace that Ukraine's allies needed to increase their support as the war entered its third year.
"We have seen hardening on the part of Russia, especially these past few months," he said.
He said this was shown on February 16 with the death in custody of President Vladimir Putin's top opponent Alexei Navalny, but also in a hardening of Russia's positions on the frontlines, where it was planning new attacks.
Mr Macron said a "leap" was needed from the West in its approach that "takes into account the transformation of the threat from a military and strategic point of view".
The conference indicated his eagerness to be seen as a European champion of Ukraine's cause, amid growing fears that American support could wane in coming months.
"Together we must ensure that Putin cannot destroy our achievements and cannot expand his aggression to other nations," Ukrainian President Volodymyr Zelenskyy said in a brief video message to the conference.
Mr Zelenskyy on Sunday told reporters that leaders in Europe realised "how dangerous" the war is for "the whole of Europe".
"I think they have realised that Putin will continue this war," he said.
Western officials acknowledge that Russia could gain the upper hand in the conflict this year as Ukraine runs out of weapons and ammunition.
Ukrainian Defence Minister Rustem Umerov said on Sunday that half of the western military aid pledged to Kyiv is delivered late, noting that "commitment does not constitute delivery".
Mr Zelenskyy said 31,000 Ukrainian soldiers had been killed in the war with Russia.
A French presidential official said the meeting must contradict any "impression that things are falling apart" after Ukraine's setbacks on the battlefield.
"We want to send a clear message to Putin that he will not prevail in Ukraine," the official said.
There are growing doubts about the viability of long-term US backing for Ukraine as a new aid package struggles to find legislative approval and Donald Trump, who has indicated opposition to further support for Kyiv, seeks to return to the presidency in elections this year.
A vote by Hungary's Parliament on Monday clearing the final obstacle for Sweden to join Nato was received with relief by Ukraine's allies.
Mr Macron congratulated Sweden, while Mr Scholz called the move "a win for all of us" and British Prime Minister Rishi Sunak hailed a "historic day" for the military alliance.
